This invention relates to a device for detecting the transit of a mobile member through a reference position of the kind comprising a photo-electric detection sensor made in a plate of semi-conductor material and able to produce a signal dependent on its illumination, means able to produce a light beam for illuminating the detection sensor, means associated with the mobile member and disposed in the path of the light beam for modifying the illumination of the detection sensor in dependence on the position of the mobile member, means for producing a reference signal, and means for producing a detection signal by comparing the signal produced by the detection sensor with the reference signal.
A device of this kind is described e.g. in U.S. Pat. No. 4,420,263, in a case where it is used to detect the transit of one of the hands of a timepiece through a reference position, for instance the position occupied by the hours hand at noon and at midnight.
The signal produced by this device is used by a circuit which periodically compares the actual position of this hand with the position it ought to have and which corrects this actual position as required. This circuit will not be described here.
The detection device described in the above mentioned U.S. Pat. No. 4,420,263 comprises a mobile disc that is mechanically connected to the hand whose transit through the reference position is to be detected. This disc rotates about its axis at the same mean speed as this hand.
This device also comprises a luminescent diode and a photo-transistor that are located, in a first constructional form, on opposite sides of the disc. The latter is opaque and is formed with a hole which is so disposed that light emitted by the luminescent diode will reach the photo-transistor only when the hand is in the reference position.
In another constructional form, also described in U.S. Pat. No. 4,420,263, the luminescent diode and the photo-transistor are disposed on the same side of the device further comprises a stationary mirror disposed opposite the diode and the transistor on the other side of the disc. The disc is also formed with a hole, and when the hand is in its reference position, light emitted by the luminescent diode reaches the photo-transistor after having been reflected by the mirror.
In the device described in U.S. Pat. No. 4,420,263, the photo-transistor is connected to the circuit that uses its signal by a shaping circuit whose nature and operation are not disclosed.
It is however obvious to the man of the art that such a shaping circuit includes a comparator which compares the signal supplied by the photo-transistor with a reference signal and which issues on its output a high or low logic signal depending on the outcome of this comparison.
In some cases, the signal supplied by the photo-transistor is constituted by the voltage across the terminals of the photo-transistor or across the terminals of a component series-connected therewith and in other cases by the current flowing through the photo-transistor. The reference signal is of course of the same nature as the signal supplied by the photo-transistor, i.e. it is constituted either by a reference voltage or by a reference current.
It is also well-known to the man of the art that, in some cases, the value of the reference signal is determined by the characteristics of one or more components of the comparator itself, e.g. by the threshold voltage of one of the transistors it comprises, and that, in other cases, this reference signal is supplied by a suitable source, external to the comparator. In all cases, the reference signal has a fixed value.
It is well-known too that the characteristics of photo-transistors of a certain type, in particular the value of the signal they produce in given conditions of use in dependence on the intensity of the light they receive, vary to a considerable extent from one specimen to another, and, in the case of each specimen, with time. The same is also true for any kind of photo-electric sensor.
These variations are in general so large that the characteristics of each sensor must be measured individually before it is fitted in the detection device to make sure the latter will work properly.
Such individual measurement of the characteristics of these sensors is very costly. Besides, it does not even ensure the device will work correctly for a reasonably long period of time since these characteristics may vary with time to a considerable and unforeseeable extent.